The purpose of the present study is to examine the nature of apatite crystals found in the hard tissue of conodont fossils. The tooth apparatus of conodont fossils (Yokokurayama Formation, Silurian, Kochi, Japan, Contact Beechweed Limestone, Devonian, Indiana, USA, Grassy Creek formation, Upper Devonian, Missori, USA, and Hushpuckney shale, Carboniferous, Missouri, USA) were used in this study. The specimens were observed and analyzed using a stereoscopy, a scanning electron microscopy, a transmission electron microscopy, an electron-probe microanalyzer, a laser Raman microprobe spectrometry, and an x-ray diffraction method. The backscattered electron image of SEM observation revealed that the crystals were highly calcified and needle-shaped. The surface layer was more calcified than the inner layer. The small tubules were observed in the inner layer. The hard tissue of the condont consisted of two layers of differing crystal size. Under higher magnification, the crystals were observed in the lattice of (100) and the central dark lines were not present. Ca, P, and F were detected in the crystal using the EPMA. The Ca/P ratio was from 1.60 to 1.76. The weight % F was from 3.203±0.646 to 5.456±0.185. By Raman spectrum analysis, the peak of 965 cm―1 was detected, which was from PO43―. Our results indicate that the apatite crystal in conodont was not hydroxyaptite but was fluoraptite. It is considered that the surface layer of conodont was enameloid and the inner layer was orthodentin or osteodentin. This study was performed under the cooperative research program of Center for Advanced Marine Core Research (CMCR), Kochi University (06B004, 07A005, 07B020, 08A004, 08B004, 09A008, 09B008). This work was supported by (JPPS) KAKENHI (20592168).
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